1. Field of the Invention
The present invention relates to a document image scanning device which scans an image on both sides of an original document while transporting the original document by an Automatic Document Feeder (ADF) which operates by one motor in an image forming device such as a facsimile machine and a copying machine (including a Multi-Function Peripheral (MFP) of these machines).
2. Description of Related Art
In general, an electrophotographic copying machine and a facsimile machine include an ADF which separates original documents set on a document tray one sheet at a time and feeds each original document toward a scanning unit. In such an ADF, a separating roller and a plurality of transportation rollers are disposed along a document transportation path. The separating roller separates and feeds the original documents fed from the document tray one sheet at a time. The transportation rollers are provided downstream of the separating roller. The transportation rollers transport the original documents toward the scanning unit and discharge the original documents that passed the scanning unit onto a discharge tray. In a known duplex scanning device, two scanning units are provided along a document transportation path. The known duplex scanning device scans an image on both sides (a first side and a second side) of an original document by transporting the original document once through the document transportation path. To downsize such a duplex scanning device, the document transportation path is formed in a sideways letter-U shape so that a discharge tray is located below a document tray. The original document is guided in a U-turn manner from the document tray to the discharge tray. In such a scanning device having the U-turn shaped document transportation path, each of the rollers is provided in proximity to one another. Therefore, without using a large-scaled drive transmitting mechanism, the rollers can be driven by one motor.
As described above, in a general document image scanning device, the scanning unit is provided along the transportation path. A driving force is transmitted from one motor to transportation systems from a picking up of an original document via the scanning unit to a discharging of the original document. In such a document image scanning device, a drive transmitting system of the transportation system of the original document leading to the scanning unit and a drive transmitting system of the transportation system at front and back of the scanning unit may be linked to one another by the same system. If the drive transmitting systems are linked to one another, while the original document passes the scanning unit, a vibration based on a fluctuation of a transportation load or a backlash of a gear or the like of the transportation system upstream of the scanning unit is transmitted to the transportation system of the scanning unit via the drive transmitting system such as a gear and a belt. The fluctuation of the transportation load and the vibration influence an image scanning process and cause a distortion of a scanned image.
In a first conventional document image scanning device, a distance from a document separator to a scanning unit is at least one half the length of a maximum original document (in case the maximum original document is A3 sized paper, 210 mm or longer) so that the size of each original document can be detected before a scanning process even when original documents of various sizes are placed on a document tray. Even in such a document image scanning device, at an instant when a trailing edge of a long original document such as an A3 size or a B4 size original document departs from the document separator, a transportation load vanishes. Accordingly, the transportation system of the scanning unit is influenced. In particular, in case two scanning units are disposed along the transportation path, a fluctuation of the transportation load becomes large at a second scanning unit. In case of an original document being smaller than a landscape A4 size original document, at an instant when the trailing edge of the original document departs from the document separator and a next original document is fed into the document separator, the fluctuation of the load is generated in the same manner. The fluctuation is transmitted to the transportation system of the scanning units. In consideration of the above-described drawbacks, a second conventional document image scanning device divides the transportation system from the picking up of the original document via the scanning unit to the discharging of the original document into two transportation systems. One transportation system leads to the proximity of the scanning unit. Another transportation system includes transportation at the front and back of the scanning unit and the discharging of the original document. The two transportation systems are driven via two drive transmitting systems. The two drive transmitting systems are distributed at an output shaft of a motor.
In the second conventional document image scanning device, the two drive transmitting systems are distributed at the output shaft of the motor. Therefore, the fluctuation of the transportation load and the vibration or the like of the transportation system leading to the proximity of the scanning unit do not influence the transportation system at the front and the back of the scanning unit. As a result, the scanned image is not distorted as described above and an image with a high image quality can be formed. However, in the second conventional document image scanning device, the scanning units are not provided at two positions along the transportation path so as to scan an image on both sides of the original document by transporting the original document once through the transportation path. In addition, the second conventional document image scanning device does not consider the influence of the fluctuation of the transportation load in the transportation system of the original document at the image scanning unit for scanning an image on a second side of the original document.